Sealed lead-acid storage cells, in which the electrolyte is completely absorbed within the pores of the positive and negative electrodes and within foraminous separator structures lying between the positive and negative electrodes, are known to the battery art. Such constructions are described, for example, in U.S. Pat. No. 3,862,861 (McClelland et al) and in U.S. Pat. No. 4,421,832 (Uba). It is a characteristic of these cells that there is not sufficient electrolyte present to fully utilize the active material of the positive and negative electrodes and, for this reason, such devices are termed "Starved Electrolyte" cells. It has been found that such cells may be operated over long periods of time without a significant loss of the water component of the electrolyte, and thus may be operated as completely sealed units. Such devices may also be termed "Oxygen Recombining " cells.
It is well known to the art that, in order to store a specific amount of electrical energy in a lead-acid storage cell, specific quantities of each of three types of materials will be required. These materials are Positive Active Material, Negative Active Material, and Electrolyte. The relationships and interactions between these three types of materials have been fully discussed in standard texts; one such reference is "Storage Batteries" by George Vinal (see 2nd Ed., Ch. 4).
In practice it is not possible to fully utilize the positive and negative active materials in a lead-acid cell. Active material utilization is dependent upon the rate of discharge. At low discharge rates (such as those lasting ten hours or more) active material utilization may be several times as great as that experienced at higher discharge rates (such as an engine "cranking" rate, which may last for as little as five to ten seconds). Thus cells designed for low discharge rates require substantially more electrolyte than do cells designed for high discharge rates. Cells built according to the teachings of McClelland et al may be suitable for high discharge rates, but are less than satisfactory when used at low discharge because of their lack of sufficient electrolyte.
In general, for a good low rate capability, the volume of electrolyte needed to fully utilize one "volume unit " of positive electrode (grid plus active material) will be approximately three "volume units," including the electrolyte required by the associated negative electrode or electrodes, and including the electrolyte contained in a suitable separator material. Thus, if the electrolyte were considered to be present in "layers," one on each side of the positive electrode and having the same height and width as the positive electrode, the combined thickness of the electrolyte "layer" would be three times as great as the thickness of the positive electrode. It is pointed out that this relationship will be dependent upon the specific gravity of the electrolyte; the above figures assume a standard specific gravity of 1.275. In "open" cells, wherein the electrolyte is not intended to be totally absorbed by the electrodes and separators, it is common to store considerable amount of "free" electrolyte in a space above the electrodes and separators; this approach cannot be tolerated in a sealed cell because oxygen transport between the positive and negative electrodes will be severely inhibited by the free electrolyte.
One of the most important factors governing the usable life of a lead-acid cell is the cross-sectional area of the bars of the grid structure of the positive electrode. Thicker bars (and thus thicker electrodes) usually result in longer-lived electrodes. Thus cells designed for low discharge rate applications commonly include positive electrodes ranging from approximately 4 mm to approximately 1O mm in thickness. In cells designed according to the teachings of McClelland et al, wherein all electrolyte is to be absorbed in the separators and electrodes, and using the three-to-one thickness ratio above, it will be seen that the total thickness of the electrolyte layers should range between 6 mm and 15 mm on each side of the positive electrode.
However, there are important reasons to keep the electrolyte layers as thin as possible. First, the electrical resistance of the electrolyte layers will be relatively high in comparison to that of the electrodes. This resistance is directly related to the thickness of the electrolyte layer. During discharge this resistance will consume electrical energy and will result in a lowered voltage at the cell terminals. This loss of power will be most noticeable at high rates of discharge, as can be seen from a consideration of Ohm's Law. The above consideration is an important factor in the design of any storage cell.
A second reason to keep the electrolyte layer as thin as possible is of particular importance in so-called "oxygen recombining" cells. In such cells the rate of oxygen recombination is inversely related to the distance between the positive and negative electrodes. Oxygen gas, which may be evolved at the positive electrodes when the cell approaches a fully-charged state, is transported to the negative electrodes where it is recombined with hydrogen and returned to the electrolyte as water. Oxygen will continue to be evolved and, likewise, will continue to be recombined as the charging process continues. It is this feature that enables an "Oxygen Recombining" cell to operate over its lifetime without an appreciable loss of water.
The aforementioned "rate of oxygen recombination" determines the amount of electrical current which an oxygen recombining cell will accept on a continuous basis after the cell has reached a fully-charged state, without a loss of water. It is desirable to design the cell such that this rate is as high as possible and, therefore, the positive and negative electrodes should be as close together as possible in order to provide a very thin electrolyte layer. In practice it has been found that the thickness of the electrolyte layer should be in a range between 1/1O mm to 3 mm. Above 3 mm the rate of oxygen recombination may be prohibitively low.
Two mechanisms for the transport of oxygen from the positive electrodes to the negative electrodes have been observed. In the first mechanism, oxygen evolved at the positive electrodes becomes dissolved in the electrolyte and is carried to the negative electrodes by the natural convective currents which occur within the cell. In the second mechanism, oxygen evolved at the positive electrodes flows directly to the negative electrodes in the gaseous phase, without the intermediate "solution" phase. In this case it is necessary to provide gas passages of some sort in order to permit the gas to flow. This may be done by only partially filling the cell with electrolyte, such that the electrolyte layer does not extend upward to the tops of the electrodes.
However, it is necessary, in order for the oxygen to be recombined at the negative electrodes, to keep the reactive areas of the electrodes damp and also to provide an electrolyte path between positive and negative electrodes. Therefore an absorbant separator material, for example glass wool or nonwoven polyester, is placed between the positive and negative electrodes. This material acts as a wick and, when less than fully saturated, provides both the desirable moist "bridge" between the electrodes and a pore structure which permits a flow of gaseous oxygen from the positive electrodes to the negative electrodes.
Quantitatively the second mechanism described above provides a far higher rate of oxygen recombination than does the first. Both mechanisms, however, are inversely related to the distance between the electrodes.
Unsealed lead-acid storage cells are known to the art in which two thinner-than-normal positive electrode portions have been placed side by side so as to constitute a single, thick positive electrode. In such cells an electrolyte supply extending above the tops of the electrodes has been used, as well as some electrolyte which lies between the two positive electrode portions.
In summary, present teachings fail to describe a sealed lead-acid cell construction in which full use is made of the positive and negative active materials during low-rate discharges, while still maintaining a rate of oxygen recombination sufficiently high so as to preclude any substantial water loss.